1. Field of the Invention
The invention relates to an adsorptive separation process for removing H.sub.2 S and HCl from a catalytic reformer offgas to make the offgas useful as a feed for a steam reforming process.
2. Description of the Prior Art
Steam reforming is a process that is used to convert hydrocarbons into a hydrogen-rich gas. It is commonly used to make synthesis gas for ammonia manufacture. Most ammonia synthesis based on steam reforming uses a light hydrocarbon-containing gas stream as a feed. The feedstock is first treated to remove contaminants that would poison the reforming catalyst or other catalysts used in the synthesis. The purified feedstock is then mixed with steam and reformed over a reforming catalyst, usually nickel supported on alumina. This reforming step is called "primary reforming" and involves the following chemical reaction: EQU C.sub.n H.sub.(2n+2) +nH.sub.2 O.fwdarw.nCO+(2n+1) H.sub.2
After the primary reforming step, air is added in a secondary reforming step, also usually carried out over a nickel-alumina catalyst, where the oxygen in the air is consumed. The nitrogen in the air provides the nitrogen for the ammonia synthesis. If the steam reforming is being used to make hydrogen not intended for ammonia synthesis, the secondary reforming will usually be eliminated. A shift conversion is then carried out on the reformed gas to produce more hydrogen and convert carbon monoxide to carbon dioxide. The shift conversion involves the following chemical reaction: EQU CO+H.sub.2 O.fwdarw.H.sub.2 +CO.sub.2
The shift conversion is usually carried out in two stages. The first is at a high temperature usually over an iron oxide catalyst and the second at a low temperature usually over a catalyst comprising copper oxide supported on zinc oxide and alumina. After the shift conversion, the bulk of the CO.sub.2 is removed from the gas by scrubbing. The remaining quantities of carbon oxides are removed in a methanation step and the synthesis gas is reacted and compressed into ammonia.
The catalysts used in the reforming and shift conversion are extremely sensitive to poisoning by sulfur and chlorine. Therefore, it is essential that these contaminants be substantially removed from the gas feedstock, usually to levels below about 0.5 ppm and preferably to levels below 0.1 ppm, before the feedstock is charged to the process. Several methods have been used or suggested for removing sulfur compounds, including H.sub.2 S, and HCl from feedstock to steam reforming processes. The two most common methods for removing H.sub.2 S are low temperature adsorption by a bed of activated carbon and high temperature adsorption by a bed of zinc oxide. Activated carbon is not effective for adsorbing HCl. Also, it must be regenerated frequently by taking the bed out of service and stripping it with steam or hot gases. Zinc oxide removes H.sub.2 S and to some extent chlorine at elevated temperatures. Zinc oxide beds are usually designed to adsorb H.sub.2 S to the extent that sulfur constitutes about 20% of the bed weight. Zinc oxide beds are not regenerable.
Molecular sieves have also been used to remove both H.sub.2 S and HCl from steam reforming feedgas. In addition, they have been disclosed as being useful to selectively remove HCl or H.sub.2 S from gases in other processes. See U.S. Pat. Nos. 3,001,607, 3,078,634, 3,078,640, and 3,197,942.